Provide a method to do a no-op subscribe to a StreamMessage

[jrhee17]

Credit to @trustin for suggesting this idea 👍

Some users may want to use StreamMessage#peek to perform some options, but not want the entire message to be in memory (as is done by StreamMessage#collect). The functionality may be similar to Flux#subscribe

In order to achieve this, users may need to do something like the following which can be cumbersome
e.g.

streamMessage.peek(i -> {/* do something */}).subscribe(new Subscriber<Integer>() {
    @Override
    public void onSubscribe(Subscription s) {
        s.request(Long.MAX_VALUE);
    }

    @Override
    public void onNext(Integer integer) {
        // don't keep the messages in memory
    }

    @Override
    public void onError(Throwable t) {}

    @Override
    public void onComplete() {}
});

We could add a method to StreamMessage that performs the above in a convenient matter.

StreamMessage.java

default CompletableFuture<Void> drain() {
    ...
}

[wooseongshin]

Hello @jrhee17 😄 I would like to handle this issue. May I try it?

[jrhee17]

All yours @wooseongshin ! Thanks 👍

[trustin]

How about naming the method as subscribe(), just like Reactor did?

[wooseongshin]

Thank you for the good suggestion @trustin I'll apply it like that!

Work is delayed due to Corona 🤧 I will try to solve it as soon as possible !